How Kidney Stones Form, Cause Pain, and Are Treated

A Pain Ranked Alongside Childbirth

Kidney stone pain — renal colic — is frequently described by those who have experienced both as comparable in intensity to labor contractions. The stone itself is not what causes pain; it is the passage of a crystal through the ureter, a tube just 3–4 millimeters in diameter, that creates the characteristic waves of severe, colicky flank pain radiating to the groin. Approximately 1 in 11 Americans will develop a kidney stone in their lifetime, according to the National Kidney Foundation, with rates having more than doubled over the past three decades, partly due to rising rates of obesity, dehydration, and dietary changes. Once a stone occurs, the 10-year recurrence rate without preventive intervention exceeds 50%.

Crystal Formation in the Collecting System

Kidney stones form when urine becomes supersaturated with mineral solutes that precipitate into crystals. Urine normally contains inhibitors of crystallization — citrate, magnesium, and nephrocalcin — that keep minerals dissolved. When the concentration of stone-forming minerals exceeds the solubility product and inhibitors are insufficient, nucleation begins: ions aggregate around a seed crystal, which grows over weeks to months into a stone.

The renal papillae — the tips of the renal medulla where urine is concentrated — are the primary sites of stone nucleation. Randall's plaques, subepithelial deposits of calcium phosphate at the renal papilla, serve as the attachment points for calcium oxalate stones in the majority of cases studied with endoscopy.

Stone Composition Types

Why Calcium Oxalate Dominates

Calcium oxalate is insoluble across a wide range of urinary pH. High oxalate excretion can result from excess dietary oxalate (found in spinach, nuts, chocolate, and tea), fat malabsorption (which causes free fatty acids to bind calcium, leaving oxalate unbound in the gut lumen for absorption), or rare primary hyperoxaluria — genetic defects in oxalate metabolism. Just 24 hours of severe dehydration can raise urinary oxalate and calcium concentrations enough to initiate crystallization.

Uric Acid Stones: A Growing Type

Uric acid stones form in acidic urine (pH below 5.5). Unlike calcium stones, they are radiolucent — invisible on plain X-ray but visible on CT and ultrasound. They are strongly associated with metabolic syndrome, type 2 diabetes, and gout. Uric acid stones can sometimes be dissolved non-surgically by alkalinizing the urine with potassium citrate to a pH of 6.5–7.0 — the only stone type amenable to medical dissolution.

The Pain of Passage

Pain begins when a stone leaves the renal pelvis and enters the ureter. Ureteral peristalsis — normally the gentle wave that moves urine toward the bladder — contracts forcefully against the obstruction, producing severe spasms. Proximal ureteral obstruction causes flank pain. Mid-ureteral stones cause pain radiating to the ipsilateral lower quadrant. Stones near the ureterovesical junction produce urinary urgency and frequency resembling bladder infection.

• Stones below 5 mm pass spontaneously in approximately 90% of cases, typically within 4 weeks
• Stones 5–10 mm pass spontaneously in roughly 50% of cases
• Stones above 10 mm rarely pass without intervention
• Alpha-blockers (tamsulosin) relax ureteral smooth muscle and increase spontaneous passage rates for stones 5–10 mm

Diagnostic Approach

Non-contrast CT of the abdomen and pelvis is the gold standard for diagnosis, detecting virtually all stone types regardless of composition or location. Ultrasound is the preferred first-line imaging in pregnancy (to avoid radiation) and children. Plain abdominal X-ray (KUB) detects calcium stones but misses uric acid and cystine stones. Laboratory workup — urinalysis, serum electrolytes, BMP, and uric acid — guides immediate management.

Treatment Options

Prevention: The Critical Long-Term Strategy

Recurrence prevention depends on stone type, identified through 24-hour urine collection analysis. General measures applicable to most stone formers include:

• High fluid intake: target urine output exceeding 2.5 liters per day; urine should be pale yellow
• Dietary sodium restriction: high sodium increases urinary calcium excretion — limiting sodium to 2,300 mg/day reduces hypercalciuria
• Adequate dietary calcium: paradoxically, low dietary calcium increases oxalate absorption; 1,000–1,200 mg/day through food (not supplements) is recommended
• Citrate supplementation (potassium citrate): citrate binds calcium in urine, reducing crystallization tendency and alkalinizing urine
• Reduce animal protein: purines in meat increase uric acid production and urinary acidity

This article is for informational purposes only. Consult a qualified healthcare professional for medical advice.